Enhanced Photocell with sample and hold amplifier

ABSTRACT

Under an embodiment, an apparatus includes an analog photocell; a sample and hold amplifier, with a first input to the sample and hold amplifier being a charge from the analog photocell and a second input to the sample and hold amplifier being a reference voltage; and an analog to digital converter, the analog to digital converter converting the output of the sample and hold amplifier to a digital value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of image processing. More specifically, the invention relates to image or motion video compression.

2. Description of the Related Art

In the current state of the art, image capture devices, those devices which represent an environment or scene by electrical signals that are proportional to the color and/or intensity of light present in the scene, are often manufactured and designed using CCD (Charge Coupled Device) technology. A CCD image capture device utilizes small photocells to generate electrical signals that are related to the incident light from the scene that strikes the photocells. The imaging device contains a two-dimensional array of such photocells such that a series of signals across an entire focused upon scene can be captured and stored. More recently, CMOS (Complementary Metal Oxide Semiconductor) imager devices have been developed which function to provide the same sort of output signals that CCD devices to but often at a lower cost and complexity. Examples of common imaging devices include scanners, motion video cameras and digital still cameras.

Many of these imaging devices, whether based on CMOS or CCD technology, are designed to be compliant with a transmission standard known as NTSC (National Television Systems Committee). One stricture of NTSC is that an image should be transmitted not in successive scan rows (of the imager array), but with odd rows transmitted separately from the even rows. This process of separating odd and even rows is commonly referred to as an interlaced scan. An NTSC signal has light intensity information encoded as analog voltage levels, color information encoded in the phase and amplitude of a color carrier frequency and so on. When an NTSC signal is forwarded for image processing to a computer system, the computer system utilizes a signal converter to transform the analog encoded information into luminance and chrominance digital values for the transmitted image. The most commonly used luminance-chrominance format for the digital representation of images is known as YCrCb (a digital color system referred to as the CCIR (International Consultative Committee on Broadcasting) 601 color space). The conversion from NTSC to YCrCb is serial in nature and due to the serial processing character of most commercially available microprocessors. Subsequent image processing is also performed in serial. One notable exception to the predominance of serial data processing is Intel's MMX(TM) technology based processors which use SIMD (Single Instruction Multiple Data) processing. To complement the use of such processors in conjunction with imaging devices, it would be useful to have parallel processing of the cells used to capture digital values. Further, a key factor in the practical application of the digital photocell is that the relatively long integration times of the analog photocell portion allows the use of a relatively slow, but therefore simple method of digitization. For motion video, which involves certain inherently serial operations such per-pixel difference calculations (where the difference between pixels and/or frames rather than the original values are encoded), it is useful to implement an architecture that allows such calculations to be performed on the imaging device rather than strictly through a host processor.

SUMMARY OF THE INVENTION

What is disclosed is an apparatus comprising an analog photocell adapted to capture light energy incident upon it as an analog signal, a sample-and-hold amplifier coupled to the photocell and adapted to store the analog signal and a digital converter coupled to the amplifier, the converter transforming the analog signal into a digital value, the value proportional to the amount of the light energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the method and apparatus for the present invention will be apparent from the following description in which:

FIG. 1 illustrates a digital photocell utilized in the invention.

FIG. 2 is a simplified block diagram of a conventional serial imager.

FIG. 3 illustrates an architecture for more efficient image differencing.

FIG. 4 illustrates one embodiment of the invention.

FIG. 5 illustrates a per-pixel analog difference engine according to an embodiment of the invention.

FIG. 6 illustrates a per-pixel digital difference engine according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the invention involves enhancing each photocell that is used in the imaging array to capture a scene. Rather than using a purely analog photocell it may be advantageous, in devices manufactured from CMOS technology, to utilize a combination of analog and digital signaling. An analog photocell can be embedded, according to one embodiment of the invention, with conversion circuitry to create a digital photocell. The digital photocell will convert the analog signal generated by incident light into a digital code, allowing it to image processed in the digital domain. An array of such digital photocells can be used, as shown in FIG. 6, to implement a digital image processing system on the imaging device.

FIG. 1 illustrates a digital photocell utilized in the invention.

An analog photocell 110 captures the light energy incident upon it from the scene being imaged. The analog photocell 110 operates according to an integration time, T, which varies according to ambient light condition. The integration time is less than the interval needed to saturate the photocell. The charge accumulated at the photocell is input to a sample and hold amplifier 120. When the photocell discharges its charge, a counter 140 is reset and begins counting for the next integration period. The counter 140 is driven by a voltage control oscillator (VCO) 130. VCO 130 has an input level set by a previously acquired charge that has been stored in the sample and hold amplifier 120. VCO 130 controls the speed at which the counter 140 increases. The greater the light intensity at the analog photocell the faster the counter 140 will be driven by VCO 130. During an integration period for a particular charge, counter 140 is counting up, and before reset, its value is sent to a register 150. The digital value in register 150, which is also the final value of counter 140, reflects the intensity value of the pixel in the previous integration period. For a motion imaging system, register 150 contains the pixel of one “frame” in the imaging. The photoelectric charge representative of the next frame is in sample and hold circuit 120 while the counter 140 is generating the digital value representative of the next frame. The register 150 holds the pixel value until it is output as part of the image or for further processing. Each of the digital photocells that compose the imager pixel array may be regulated using the same timing and control sequence since the photocells act in parallel, outputting an entire frame periodically.

To ensure that the dynamic range of the counter matches the dynamic range of the photocells, the sample and hold amplifier 120 can be equipped to scale the input to VCO 130 as appropriate. The dynamic range may be mismatched due to differing ambient light levels in the scene being captured. The variance in integration period that may result from a change in ambient light of the scene ensures that the captured image has the proper contrast. To adjust the dynamic range of the VCO 130 to match the analog photocell, a global scaling voltage 160 can be applied to the sample and hold amplifier of each digital photocell in the array which uniformly adapts the VCO component in each photocell to have a dynamic r e consistent with the present ambient light conditions. The enhanced digital photocell of FIG. 1 may be utilized in a serial imaging device, or for use in parallel image processing architectures.

FIG. 2 is a simplified block diagram of a conventional serial imager.

As noted above, motion video compression such as MPEG, utilizes a differencing approach to encode successive frames of image data. After two frames are captured or imaged completely, the difference between corresponding pixels is computed and this difference is then encoded. This allows highly correlated or redundant image features to be encoded using the fewest number of bits. For instance, in a video-conferencing session, the background of the captured image may change only slightly or not at all from one frame instance to the next. In this case, rather than transmitting the entire background portion at a successive frame, just the pixel variance between frames could be transmitted. In FIG. 2, the serial imager utilized in CCD imaging devices would shift out pixel information for an entire frame and then another entire frame before the first difference frame could be computed. This conventional method required thus the capturing and storing of two (or more) entire image frames to generate a third frame representing the difference. The first frame, a “key” frame is captured and digitized, as is a successive frame. The digitized frames are then differed to generate a difference frame. To reduce the delay and computational load in conventional image differencing apparatus, an architecture similar to that of FIG. 3 may be employed.

FIG. 3 illustrates an architecture for more efficient image differencing. The conventional design of imaging devices is to perform differencing of successive captured frames after they are captured and digitized. To improve upon this conventional design, the computational load can be reduced significantly if an analog differencing is done prior to digitizing. The parallel-shift differencing apparatus of FIG. 3 utilizes shifting to achieve the goal of generating image difference data.

Consider a set of eight exemplary analog photocells A₃₁, A₃₂, A₃₃, A₃₄, A₃₅, A₃₆, A₃₇, and A₃₈. Photocells capture analog light intensity at fixed locations in the scene. These intensity values are represented by an amount of charge that accumulates in the photocell during its integration time. The photocells A₃₁, . . . A₃₈ do not generate a digitized output as does the digital photocell unit of FIG. 1. Rather, the stored charge is passed at the end of the integration period (which is the same for all photocells in a given frame instant), to a corresponding shift cell. For each row of analog photocells, there are two rows of shift cells. One row of shift cells stores photocell outputs for a first frame (“key” frame) while a second row of shift cells stores the photocell outputs of the successive frame. Each row of shift cells outputs photocell data serially.

The row of shift cells for key frame output are designated S₃₂, S₃₄, S₃₆, S₃₈, S₄₀, S₄₂, S₄₄ and S₄₆. The row of shift cells storing output for the frame immediately succeeding the key frame are labeled S₃₁, S₃₃, S₃₅, S₃₇, S₃₉, S₄₁, S₄₃ and S₄₅. When the imaging architecture is first initialized, all shift cells store a null or zero intensity value. When the first image frame is captured, each of the analog photocells A₃₁, . . . A₃₈ will develop a charge representative of light intensity at a particular location in the scene that is incident upon the photocell. This set of signals is transferred to the row of shift cells S₃₁, S₃₃, . . . S₄₆. The row of shift cells for key frame is at that instant, unfilled. Rather than outputs this first frame of data, the architecture waits until the next frame is captured. When the next image frame is captured by analog photocells A₃₁, . . . A₃₈, the result of the previous frame is first transferred from shift cells S₃₁, S₃₃, . . . S₄₅ to the row of shift cells S₃₂, S₃₄, . . . S₄₆, respectively, as indicated in FIG. 3. Next, at the end of the integration period for the second frame, the signals are transferred from analog photocells A₃₁, . . . A₃₈ to the shift cells S₃₁, S₃₃, . . . S₄₅. At that instant, both rows of shift cells contain image frame information. The row of shift cells S₃₂, S₃₄, . . . S₄₆ which stores the first frame is shifted out. This represents a key frame output 312. Key frame output 312 is simultaneously shifted to an input differential op-amp (operational amplifier) 310.

The result of the current frame stored in shift cells S₃₁, S₃₃, . . . S₄₅, is shifted out to the other input of differential op-amp 310. Differential op-amp 310 generates an analog signal, delta frame output 314, which is the result of previous frame (key frame) subtracted from the current frame. The analog signal delta frame output 314 and key frame output 312 may both be digitized prior to storage or processing. Since a serial shifting operation will output the analog key frame and current frame outputs only pixel by pixel, the entire current frame and key frame must first be shifted to output and to differential op-amp 310. After the serial shifting operation is complete and the last of the key and current frames are output/processed, then the current frame stored in shift cells S₃₁, S₃₃, . . . S₄₅ is shifted in parallel to the row S₃₂, S₃₄, . . . S₄₆ and thus, becomes the next key frame.

The advantages of this design lie primarily in the ability to send to digitization only an analog difference frame output, rather than two entire image frames. Depending on the further down-the-line processing to be performed, the delta frame output 314 and/or the key frame output 312 may be digitized. In the conventional design, two entire frames of analog photocell information is captured, and shifted out separately, after which digitizing and differencing are performed. In the architecture of FIG. 3, both the key frame 312 and the differential for the next frame (delta frame output 314) are shifted to output simultaneously.

The embodiment of FIG. 3 still requires the shifting out of an entire key frame and difference frame, albeit simultaneously, before another frame can be captured by the analog photocell. A further improvement to this architecture is shown in FIG. 4 according to yet another embodiment of the invention.

FIG. 4 illustrates one embodiment of the invention.

In the embodiment of FIG. 4, the current frame is shifted out immediately and is also regenerated and fed back by way of an op-amp 420.

In the architecture of FIG. 4, a current image frame is captured by an array of N+1 analog photocells A₀, A₁, . . . A_(N) which are then passed in parallel to shift cells C₀, C₁, . . . C_(N), respectively. Shift cells C₀, C₁, . . . C_(N) shift out the captured frame data in a cascade (bucket-brigade) fashion from C_(N) to C₀. The current frame data is regenerated by an op-amp 420 and fed back to an array of shift cells S_(N), . . . S₀ as shown. As the regenerated current frame is fed back, the current frame is differenced against a previous frame shifted out of the array of shift cells S₀, . . . S_(N). The differencing between the current frame and the previous frame is accomplished by an op-amp 410 and produces a pixel-by-pixel difference frame output. This serial imaging system has the intended advantage of providing both the current frame and a difference frame without having to wait for an entire frame of pixel data to be captured. Ordinarily, two frames, a first frame and a second frame must both be captured before a difference frame can be generated. The architecture of FIG. 4 eliminates such a limitation on serial imaging. Op-amps 410 and 420, though not described in detail, can be designed by one of skill in the art but should have the capabilities of boosting signal integrity in the case of op-amp 420 and differencing two signals in the case of op-amp 410.

FIG. 5 illustrates a per-pixel analog difference engine according to an embodiment of the invention.

The embodiment of FIG. 5 allows the transmission of multiple difference frames based upon a key frame. The imaging apparatus would first capture and transmit at the image output the key frame where none of the pixels are differential. This key frame, shifted out via shift cells S₅₁, S₅₂, . . . S₅₄ is also fed back into an array of analog holding registers H₅₁, H₅₂, . . . H₅₄. Prior to being fed back and output, each pixel is passed to a regeneration amplifier R5 which regenerates the charge level of the pixel to avoid loss in the delay of transmission. This feedback of the current frame into holding registers H₅₁, . . . H₅₄ assures that the frame will be available as the “previous” frame when the next frame is captured. With the completed frame thus stored, it is possible to calculate a difference frame and transmit this difference at the next frame cycle.

To achieve this, the output each analog holding register H₅₁, H₅₂, H₅₃ and H₅₄ is linked to the input of a differential operational amplifier O₅₁, O₅₂, O₅₃ and O₅₄, respectively. The current frame is captured by analog photocells A₅₁, A₅₂, A₅₃ and A₅₄ whose output is passed to other input of differential operational amplifier O₅₁, O₅₂, O₅₃ and O₅₄, respectively. After the key frame is transmitted, at the output, each subsequent frame may be transmitted as the difference relative to the previous frame or key frame as computed by amplifiers O₅₁, O₅₂, O₅₃ and O₅₄. Any number of subsequent “difference” frames may be transmitted to the output until the next key frame is desired. The input to each shift cell S₅₁, S₅₂, S₅₃ and S₅₄ is one of either the entire original frame captured by analog cells A₅₁, A₅₂, A₅₃ and A₅₄, respectively, (when a key frame is desired) or the difference output of the operational amplifiers O₅₁, O₅₂, O₅₃ and O₅₄, respectively.

A select signal (not shown) is sent to each of a set of analog multiplexers M₅₁, M₅₂, M₅₃ and M₅₄ which routes either the appropriate key frame data (frame A₅₁, A₅₂, A₅₃ and A₅₄, respectively) or difference frame data (O₅₁, O₅₂, O₅₃ and O₅₄) as desired by the application user to shift cells S₅₁, S₅₂, S₅₃ and S₅₄, respectively. Additional rows of shift cells and similar architecture may be linked together one after another as described.

FIG. 6 illustrates a per-pixel digital difference engine according to an embodiment of the invention.

The embodiment of FIG. 6 allows the transmission of multiple difference frames based upon a key frame. The imaging apparatus would first capture and transmit at the image output the key frame where none of the pixels are differential. The initial key frame which is captured by digital photocells (pixels) D61, D62, D63 and D64, is output on output bus 600. Simultaneously, the digital pixels D61, D62, D63 and D64 is fed to a series of digital holding registers H61, H62, H63 and H64, respectively. This feedback of the current frame into holding registers H₆₁, . . . H₆₄ assures that the frame will be available as the “previous” frame when the next frame is captured. With the completed frame thus stored, it is possible to calculate a difference frame and transmit this difference at the next frame cycle.

To achieve this, the output each digital holding register H₆₁, H₆₂, H₆₃ and H₆₄ is linked to the input of a subtraction unit S₆₁, S₆₂, S₆₃ and S₆₄, respectively. The current frame is captured by digital photocells D₆₁, D₆₂, D₆₃ and D₆₄ whose output is passed to other input of subtraction unit S₆₁, S₆₂, S₆₃ and S₆₄, respectively. After the key frame is transmitted, at the output, each subsequent frame may be transmitted as the difference relative to the previous frame as computed by subtraction units S₆₁, S₆₂, S₆₃ and S₆₄. Any number of subsequent “difference” frames may be transmitted to the output until the next key frame is desired.

The output bus transmits one of either the key frame pixels (from D₆₁, D₆₂, D₆₃ and D₆₄) or difference frame pixels (from subtraction units S₆₁, S₆₂, S₆₃ and S₆₄) depending on what the applicant/user desires. Based upon the desired mode, key or difference, a select signal (not shown) is sent to each one of digital multiplexers M₆₁, M₆₂, M₆₃ and M₆₄ which then accordingly routes either key frame pixels (from D₆₁, D₆₂, D₆₃ and D₆₄, respectively) or difference frame pixels (from S₆₁, S₆₂, S₆₃ and S₆₄, respectively) as indicated. Additional digital outputs similar to those provided by M₆₁, M₆₂, M₆₃ and M₆₄ may be repeatedly constructed for each pixel desired.

The exemplary embodiments described herein are provided merely to illustrate the principles of the invention and should not be construed as limiting the scope of the invention. Rather, the principles of the invention may be applied to a wide range of systems to achieve the advantages described herein and to achieve other advantages or to satisfy other objectives as well. 

What is claimed is:
 1. An apparatus comprising: an analog photocell; a sample and hold amplifier, a first input to the sample and hold amplifier being an output from the analog photocell, a second input to the sample and hold amplifier being a reference voltage, the sample and hold amplifier producing an output that is a scaled version of the output of the analog photocell, the scaling of the output being controlled by the reference voltage; and an analog to digital converter, the analog to digital converter converting the output of the sample and hold amplifier to a digital value, the scaled version of the output of the analog photocell being chosen to match a dynamic range of the analog photocell with a dynamic range of the analog to digital converter and being based, at least in part, on ambient light conditions.
 2. The apparatus of claim 1, wherein the analog to digital converter comprises: a voltage controlled oscillator, an input of the voltage controller oscillator being a output from the sample and hold amplifier; and a counter, the counter being driven by an output of the voltage controlled oscillator.
 3. The apparatus of claim 2, further comprising a memory, the memory storing output of the counter.
 4. The apparatus of claim 3, wherein the counter is reset after a certain period of time.
 5. The apparatus of claim 4, wherein the period of time is an integration time for the analog photocell.
 6. A method comprising: inputting a charge of a analog photocell to a sample and hold amplifier; inputting a reference voltage to the sample and hold amplifier; modifying the scale of the analog photocell charge using the sample and hold amplifier, the modification of the scale of the analog photocell charge being controlled by the reference voltage; and converting an output of the sample and hold amplifier to a digital value, the scale of the analog photocell charge being modified by the sample and hold amplifier to match a dynamic range of the analog photocell to a dynamic range appropriate for converting the output of the sample and hold amplifier to a digital value, and the scale of the analog photocell charge being based, at least in part, on ambient light conditions.
 7. The method of claim 6, wherein converting the output of the sample and hold amplifier to a digital value comprises: applying an output of th sample and hold amplifier to a voltage controlled oscillator; and driving a counter using the output of the voltage controlled oscillator.
 8. The method of claim 7, wherein a count from the counter is proportional to the intensity of light on the analog photocell during a previous integration time period for the photocell.
 9. The method of claim 8, further comprising storing a count from the counter in a register.
 10. The method of claim 9, further comprising resetting the counter after the passage of the integration time period for the photocell.
 11. An digital photocell comprising: an analog photocell; a sample and hold amplifier, a first input of the sample and hold amplifier being an output of the analog photocell and a second input of the sample and hold amplifier being a reference voltage; a voltage controlled oscillator, an input to the voltage controlled oscillator being an output of the ample and hold amplifier, the sample and hold amplifier to scale the input to the voltage controlled oscillator, the scaling of the input to the voltage controlled oscillator being controlled by the reference voltage, the reference voltage to be set to match the scale of the input to a dynamic range of the voltage controlled oscillator input to the voltage controlled oscillator based at least in part on ambient light levels; a counter, a speed at which the counter operates being controlled by an output of the voltage controlled oscillator; and a register, the register storing an output of the counter.
 12. The digital photocell of claim 11, wherein the counter counts for a specified time period and wherein the counter is reset at the end of the time period.
 13. The digital photocell of claim 12, wherein the time period is an integration time period for the analog photocell.
 14. The digital photocell of claim 13, wherein the output stored in the register is a digital value that reflects an intensity of light incident on the analog photocell during the previous integration time period.
 15. The digital photocell of claim 14, wherein the digital photocell is included in a pixel array.
 16. A method comprising: applying an output of a analog photocell as a first input to a sample and hold amplifier; applying a reference voltage as a second input to the sample and hold amplifier; modifying the scale of the output of the analog photocell using the sample and hold amplifier, the modification of the scale of the output of the analog photocell being controlled by the reference voltage, the scale of the analog photocell charge being modified to match a dynamic range of the analog photocell to a dynamic range appropriate for converting the output of the sample and hold amplifier, the scale of the analog photocell charge being based at least in part on ambient light conditions; and applying the output of the sample and hold amplifier to a voltage controlled oscillator; driving a counter with the output of the voltage controlled oscillator; saving a count from the counter; and resetting the counter at the conclusion of a time period.
 17. The method of claim 16, wherein the time period is an integration period of the analog photocell.
 18. The method of claim 17, wherein the count from the counter is saved in a register.
 19. The method of claim 18, wherein the count from the counter is proportional to intensity of light incident on the analog photocell. 